With the tendency of higher integration of semiconductor devices every year, resist materials and exposure techniques have been improved in response to miniturization of patterns formed on a substrate such as a semiconductor wafer (hereinafter referred to as a “wafer”), and the dimension of an opening formed in a resist mask has also been reduced.
However, as a pattern formed in a resist mask is miniaturized, the aspect ratio (which is a ratio of the depth of the opening to the width of the opening) of the opening formed in the resist mask is larger. This causes toppling of the resist mask. Thus, a multilayer resist film having high strength is used when a pattern of a high aspect ratio is necessary (See Japanese patent laid-open publication JP10-268526A, Paragraph 0002-0006; and Toshiba Review, Vol. 59 No. 8 Page 22 (2004), for example).
As shown in FIG. 6(a), a wafer 100, which is a substrate used in such a process, has a structure in which: a photoresist mask 70 having openings 75 formed therein; an SOG (Spin On Glass) film 71 formed of a coating film made of SiO2; an organic film 72 formed of an organic matter having higher strength than the photoresist mask 70; and an insulating film made of an inorganic matter such as a silicon oxide film 73 are stacked in that order on an Si layer 74 having a gate electrode (not shown) formed therein. In the wafer 100, as shown in FIG. 6(b), the SOG film 71 is etched, for example, by using plasma of CF4 gas. Then, the organic film 72 is etched and the photoresist mask 70 is removed with the use of plasma of an 02 gas, as shown in FIG. 6(c). After that, the silicon oxide film 73 is etched, the SOG film 71 is removed, and holes 76 are formed as shown in FIG. 6(c). With this layered structure, a resist mask (the organic film 72) having a pattern with a high aspect ratio can be formed on the silicon oxide film 73, i.e., an etch target film, while the possibility of toppling is reduced, whereby a fine pattern can be transferred to the silicon oxide film 73. In this process, each of the photoresist mask (film) 70, the SOG film 71, and the organic film 72 is used as an etch mask for etching an underlying film, and therefore, each of the films is made of a material different from that of an underlying film to be etched (with a high etch selectivity).
In recent years, as shown in FIG. 7(a), openings 75 in a pattern have an elliptical shape, and side wall portions of the openings 75 with a small thickness laterally extend between the openings 75, and the height of the side wall portions are large. Thus, the side wall portions are likely to be toppled. In order to reduce the size of the openings 75 formed in the photoresist mask 70, the present inventors have been studying to deposit a material on the side walls of the opening 75. To this end, it is considered that a C—H—F series gas, which is per se widely used, may be used so as to produce a deposit and reduce the size of the openings. In this case, however, the following problem occurs. That is, when a C—H—F series deposit is deposited on the photoresist mask 70 in order to reduce the dimension of the opening, the deposit also adheres to the inner wall of a processing chamber. Then, the SOG film 71 is etched, and the organic film 72 is subsequently etched by using the SOG film 71 as a mask. When the organic film 72 is etched, the deposit, which has adhered to the inner wall of the processing chamber, is etched by active species of oxygen so as to be released into the atmosphere. As the deposit includes fluorine, the SOG film 71 is etched and thus no longer function as a mask, as shown in FIGS. 7(a) and 7(b). To be more specific, the openings 75 formed in the SOG film 71 are etched so that the shape of the openings 75 is deteriorated, and the shape of the openings thus deteriorated is transferred to holes 76 formed in the silicon oxide film 73 through the organic film 72. As a result, the shape of the holes 76 is deteriorated such that vertical steaks called striations are developed in the holes 76. Thus, it is necessary that the inner walls of the processing chamber be cleaned between the etching of the SOG film 71 and the etching of the organic film 72. Therefore, the wafer 100 is required to be taken out of the processing chamber. This prevents the processes from being continuously performed in the same processing chamber.
JP2004-103925A (paragraph 0017 and 0055) discloses the use of CF4 gas, CHF3 gas, CH2F2 gas, or CH4 gas for depositing a material on a resist mask, however, fails to suggest the way of solving the foregoing problem.